how do I prevent the belt from clipping my model (I've used data transfer to transfer the weights)
A simple approach might be to build the belt geometry directly from selected and duplicated faces of the waist geometry, which would then be separated into a new object. However, this would need to be done only after binding the body mesh to the rig. This is because, whenever an object with modifiers is duplicated, the duplicate also has the same modifiers. In this case, your belt mesh will have an armature modifier on it as soon as it becomes a duplicate of the already bound body mesh. The belt mesh will need to have this modifier in order to be deformed by the rig. The belt mesh will automatically have the needed vertex groups for deformation, also, as well as fairly adequate starting weights for those vertices. What it won't have, is thickness. For that you can add a Solidify modifier. 

You may find that you still must do some weight paint tweaking afterwards to ensure no collisions between waist mesh and belt mesh. But the work is reduced somewhat by starting this way.